1. Field of the Invention
The present invention generally relates to digital communication radio receivers. More specifically, the present invention concerns compensation of frequency drift and phase noise of a local oscillator in the radio receiver.
2. Description of the Related Art
Digital communication radio receivers commonly use a radio frequency front end (RF-FE) to down convert a desired signal, modulated on some radio frequency (RF) carrier, to a lower intermediate frequency (IF) or a zero IF frequency (DC). The mixed signal can be sampled by an analog to digital converter and processed using known digital signal processing techniques to demodulate the received signal and extract transmitted information bits. In order to down mix the signal, however, the RF-FE typically generates a sinusoidal signal, which multiplies the received signal and creates the mix-down operation. This sinusoidal signal is commonly known as the local oscillator (LO) signal.
The LO is commonly generated using a voltage controlled oscillator (VCO) that is controlled by a phase locked loop (PLL). The PLL tracks a reference clock to ensure that LO has a stable frequency. The PLL circuitry, however, requires a considerable amount of power to be able to stabilize the LO. In addition, the PLL usually requires a narrow low pass filter in order to filter out noise from the reference clock and provide a low noise feedback to the VCO. Implementing such a filter in an integrated circuit RF-FE design requires considerable area for the circuit or additional external components that increases the cost of the receiver.
To address the footprint of the filter, the PLL controlled VCO is replaced with a digitally controlled oscillator (DCO) in some RF-FE receivers. The DCO output frequency can be controlled by adding or subtracting capacitance to an oscillator, which changes the oscillation frequency. A DCO circuit generally consumes less power than a PLL controlled VCO and generates less of a footprint. The DCO is, therefore, a compelling solution for LO generation, especially in IC RF-FE used in battery powered handheld devices.
The disadvantage of using DCO, however, is that the LO frequency generated by a DCO is not as stable as an LO frequency generated by a VCO controlled by a PLL. The DCO frequency can drift by a large percentage when the receiver temperature changes or when there are even small changes to the supply voltage of the DCO circuitry. The LO generated by the DCO will also have a higher phase noise than an LO generated by a PLL controlled VCO. As most digital modulated communication signals require high accuracy, low phase noise, and high stability of the LO to successfully demodulate a received signal, even a slight variance can be extremely detrimental to system operations.
Even worse, a rapid and large drift in the IF frequency, caused by the DCO generated LO drift will significantly degrade the quality of the demodulation by introducing errors in the decoded bits. For this reason, DCOs are not commonly used in an RF-FE intended for decoding digitally modulated signals, but instead used in RF-FE receivers for demodulating frequency modulated (FM) signals, where the frequency drift of the LO is not harmful for the demodulation.
There is a need in the art to digitally compensate for DCO frequency drift whereby a DCO generated LO can be used to down convert a digitally modulated signal in order to enjoy significant power and cost advantage over traditional VCO with PLL RF-FE.